By Armin de Meijere, A. Brandi, F.M. Cordero, A. Goti, T. Hirao
This paintings on small ring compounds in natural synthesis covers cycloadditions onto methylene- and alkylidenecyclopropane, and selective changes of small ring compounds in redox reactions.
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Extra info for 178 Topics in Current Chemistry: Small Ring Compounds in Organic Synthesis V
R anti-anti ~ , approach H OC, ,, 1 major diastereoisomers Fig. 3. Preferred TS trajectory for the cycloaddition of 5-substituted pyrroline N-oxides to ringsubstituted methylenecyclopropanes 46 4=. Entry I 40 =C,? , 7 days 35 °C. adduct OR 10% 83 82 8% 15% 327 84 8% 10% O_~N_.. 82 9% 6% Ref. -Spirocyclopropane isoxazolidine 324 @°. = l= O E =,. E 5" O == O o A. Goti et al. H toluene,110"C,6 h O H ~ 68% 292 1. TsNHNH2,MeOH, 60"C, 5h 2. -OH 85% (+)-Lentiginosine Scheme 47 Hydroxylated nitrones 321 and 322 afford, by subsequent thermal rearrangement of the adducts, a straightforward approach to polyhydroxylated indolizidines, inhibitors of glycosidases.
If we consider all the possible transition states with charge separation (Fig. 2) for the cycloaddition to methylenecyclopropane derivatives, it is apparent that TS-B, which holds a negative charge on a cyclopropyl carbon, and TS-D, which bears a positive charge ct to a cyclopropyl ring, are more stabilized than the regioisomeric TS-A and TS-C. They both lead to the formation of 4-spirocyclopropane isoxazolidines. When R is an electrondonating group the stabilization is much higher, leading to the exclusive formation of 4-spirocyclopropane isoxazolidines.
Adding fire to the debate, the cycloaddition can be assumed to take place through an asymmetric polarized transition state where charge separation assumes higher importance. If this is true, transition states that are able to stabilize the separate charges will be preferred. If we consider all the possible transition states with charge separation (Fig. 2) for the cycloaddition to methylenecyclopropane derivatives, it is apparent that TS-B, which holds a negative charge on a cyclopropyl carbon, and TS-D, which bears a positive charge ct to a cyclopropyl ring, are more stabilized than the regioisomeric TS-A and TS-C.